Anaerobic digestion is a well-known process used to obtain usable methane gas and other useful products from manure, waste sludge and other predominantly organic materials. Organic materials provide the substrate for anaerobic fermentation or biodegradation, and may be comprised of a wide range of organic carbon sources, such as plants and crop wastes, sewage sludge, and other refuse. Anaerobic digestion is a fermentation or biodegradation process that breaks down or degrades these carbonaceous materials to produce gases, such as methane and carbon dioxide. Anaerobic digestion utilizes consortia of microorganisms to degrade and then convert the carbonaceous material to produce gases, under certain pressure, temperature and other environmental conditions.
The United States has over 1000 Billion tons of coal resources, and more than half of this resource is low-rank coal. Coal is a heterogeneous material that consists of carbon, hydrogen, oxygen, nitrogen, sulfur and other minerals. The combustion of coal releases oxides of carbon, nitrogen and sulfur, as well as some heavy metals such as mercury, into the atmosphere. Coal combustion generates the most pollution of any fossil fuel resource. Low-rank coals have low market value commensurate with their Btu content, and also generate a large amount of pollutants, making their use increasingly unattractive for power generation.
A number of different coal conversion technologies that employ thermal and/or chemical processes have been in commercial use for many years but these processes convert coal to gases and chemicals under high pressures and temperatures with high capital and operating costs, relatively low thermodynamic efficiency, and generation of significant amounts of carbon dioxide and other gaseous emissions, and also require large amounts of water in the process with solid waste streams that must be disposed of safely.
The present invention solves these problems by a process of bioconversion of coal to much cleaner-burning methane and other useful products to increase the supply of these energy resources, to effectively utilize a fossil fuel resource that may otherwise be wasted or not used, and to employ technology that has a low environmental impact with high efficiency. Consequently, anaerobic bioconversion of coal has lower capital and operating costs, has higher thermodynamic efficiencies, and produces much less gaseous emissions and solid wastes.